Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, which are particularly suitable for use in a broadcasting television camera, a cinema camera, a video camera, and a digital still camera.
Description of the Related Art
In recent years, as an image pickup apparatus such as a television camera, a cinema camera, a video camera, or a film camera, a large format camera having features of a shallow depth of field and beautiful bokeh (blur) quality for expanding visual expression is used. As a zoom lens to be attached to the large format camera, a small and lightweight zoom lens having a high zoom ratio for securing mobility and improving flexibility in photography has been in demand. In addition, an image pickup element having a large number of pixels has been used as an image pickup element of the camera, and hence the zoom lens is required to have high performance accommodating an increased definition. As a zoom lens having the high zoom ratio, as proposed in Japanese Patent Application Laid-Open No. 2007-139858 and Japanese Patent Application Laid-Open No. 2004-085846, there has been known a zoom lens of a positive lead type in which a lens unit having a positive refractive power is arranged closest to an object side, and which includes at least four lens units as a whole.
In Japanese Patent Application Laid-Open No. 2007-139858, a zoom lens including four lens units and having an angle of field at a telephoto end of about 0.7 degrees and a zoom ratio of about 15× has been proposed.
In Japanese Patent Application Laid-Open No. 2004-085846, a zoom lens including four lens units and having an angle of field at a telephoto end of about 1.6 degrees and a zoom ratio of about 3× has been proposed.
A problem, which arises in realizing the high zoom ratio of the zoom lens, is increased variations in various aberrations accompanying zooming. In particular, when a variation in lateral chromatic aberration due to zooming is increased, it becomes difficult to satisfactorily correct the lateral chromatic aberration over an entire zoom range, and to obtain a high definition image.
A problem for the zoom lens in each of Japanese Patent Application Laid-Open No. 2007-139858 and Japanese Patent Application Laid-Open No. 2004-085846 in realizing the high zoom ratio while accommodating a large format sensor is that, because the variation in lateral chromatic aberration due to zooming is increased accompanying the increase in zoom ratio, it is difficult to satisfactorily correct the lateral chromatic aberration over the entire zoom range.
In order to satisfactorily correct the lateral chromatic aberration over the entire zoom range while realizing the high zoom ratio, it is important that a lens configuration of a second lens unit be devised to suppress the variation in lateral chromatic aberration due to zooming, and that a configuration of a lens unit closest to the image side be devised to adjust the lateral chromatic aberration at a wide angle end.